The child in question (let's call him "The Incredible Bulk") is 1/4 red dragon, 1/4 troll, 1/2 green dragon by blood, but 1/2 red dragon, 1/2 green dragon, whole troll by RAW. And The Bulk is good for trolling, if nothing else.

The reason you don't use more half-dragons is because red/green or red/black cover give you immunity to fire and acid (you could substitute gold/brass for red and copper for green/black if you want a Good character). Adding more templates adds more Level Adjustment. Which is more valuable, all three levels of Hulking Hurler or another increase to stat boosts without gaining the ability to throw a tank at people?

Might as well make them Bladerager Trolls from MMV. They have the same HD & LA, slightly better stats, immunity to mind-affecting, free full plate armor, and explode when killed.

Like I said, it was back in the 3.0 days.... He was actually fun to play at the time. We ruled that his wings HAD to be big enough to stick out of the antimagic, so he tended to stay on the ground vs spellcasting targets. Most fights went like so...

Step 1: troll charges into fray...
Step 2: enemies panic and waste a round of magic discovering his antimagic aura
Step 3: troll drops 1 or 2 opponents before being mobbed and taken down...
Step 4: the rest of the party mops up the rest of the resistance
Step 5: troll groans and stands up, asking what he missed....

DR/magic doesn't work in anti-magic, so that's not a problem. It's the fire and acid resistance that DO work, in conjunction with regeneration. You can drop him (it happened plenty, though I deliberately played him to allow that, since we were in a group) but doing lethal damage was tougher. You've got to figure out where that AMF is coming from. In his case, it was an amulet, buried in his chest, that caused him to emanate it... Honestly, his damage output was mediocre at best. He was just a great sponge.

That's why he had to have the party access his portable hole. I decided to not give him an easy way to turn his AMF off, just for fun.

Why wouldn't DR/magic work in antimagic? DR is often an extraordinary ability. DR is usually not so high though that it's the be all and end all of fights. I've wondered about DR/magic combined with Dampen Magic but it doesn't really seem worth the effort.

This answer claims that according to the errata, DR/slashing, bludgeoning, piercing, adamantine, and - are considered extraordinary, while silver, cold iron, magic and alignment are supernatural. But that does contradict the lycanthrope entry above.

Addendum: Liches, have DR/bludgeoning+magic, and it's supernatural for them.
Skeletons have DR/bludgeoning, extraordinary, while zombies have DR/slashing, extraordinary.
Astral Constructs have DR/magic, extraordinary (though it's a moot point since they're summons themselves).
The Monk's 'Perfect Self' 20th level ability gives them DR/magic...and annoyingly, isn't classified as a special quality at all (so no word on if it's extraordinary or supernatural). The barbarian's DR/- is extraordinary.
Tainted Warriors under the variant rules get DR/good that's extraordinary.
Aand...Demiliches have DR/epic+bludgeoning, which is supernatural.

Actually, searching for "Damage Reduction (Su)", it seems like only liches and vampires have supernatural damage reduction. Explicitly, anyway. I couldn't find anything on the numerous demons and devils with DR.